Edition 04/15

What about the typical sewage water smell? No chance! Visitors to the “station d’epuration” located in the district of Beggen have to pay close attention in order to realise that a wastewater treatment plant is in the neighbourhood - and one wich processes a daily water volume between 30,000 and 40,000 m3. In order to avoid bad smells emanating from the plant, the city of Luxembourg has enclosed all cleaning stages, even the biological processes. In combination with a powerful exhaust air system, the malodorous air is led through a chemical scrubber prior to delivery to the outside world. “The fans run continuously at 30 kW,” Production Manager, Luc Ley, notes pragmatically. He knows that low emissions come at a price, especially looking at the operating costs which arise from power consumption. “Therefore, for us, only the application of IE3-motors makes sense.“

More air for microorganisms

Due to EU-wide stipulated limit values, especially with regard to nitrogen and phosphorus, the municipality had to face the challenge of modernising the existing plant. Due to limited space, an increase in capacity was not the only option. “This is why we are today using fixed bed reactors,” explains Ley. The procedure works properly, but for nitrogen degradation, considerably more air is required. “We need an oxygen saturation between 7 and 8 mg/l. The standard is between 1 and 1.5 mg/l.” The high oxygen requirement results from the special cleaning procedure in the fixed bed reactors. If the air quantity is not sufficient, the microorganisms run out of breath. For the air supply, one AERZEN Delta Hybrid rotary lobe compressor D62 S is applied with each reactor. The packaged units supply a volume flow between 1,700 and 2,000 m3/h at a slight overpressure between plus 550 and 620 mbar.

Long-life operation

As part of a large-scale modernisation of the ventilation technology, the wastewater company had an AERZEN service team size up and install more powerful rotary lobe compressors. The packaged units of series Delta Hybrid have a connected load of 75 kW. However, they are operated with only a nominal power of 40 kW via the frequency converters. This means that the Delta Hybrid systems produce the required volume running at 5,000 rpm, instead of 9,000 rpm. “Today, the packaged units are designed in such a way that they get bored as far as wear and tear is concerned,” Ley notes with a smile. Although it might seem paradoxical at first, the overdimensioning provides tangible advantages, such as for longevity and maintenance intervals. The previous equipment required maintenance every three years. This has changed, as the Delta Hybrid systems only require service every five years. “Assuming a lifetime of ten years, I will save one complete maintenance sequence,” adds Ley. Another positive effect of the new units is the reduction in energy costs, thanks to the special operating principles of the Delta Hybrid units. Here, AERZEN combines the operating methods of both positive displacement blowers and screw compressors. The result is an efficient unit - the world’s first rotary lobe compressor. The economical use of results from the fact that the Delta Hybrid series calls for the Roots-Principle of isochoric compression in low pressure applications and the screw compressor principle, with its internal compression, becomes the preferred choice for its energy efficiency in higher pressure ranges. AERZEN has calculated that the rotary lobe compressor requires 15 percent less current than existing compressors. In addition, the air conveying is absolutely oil-free which is an important feature in wastewater treatment.

More than promised

The effects of this operating method in practice are reflected in the power consumption of the wastewater treatment plant in Luxembourg. “Prior to retrofitting, AERZEN promised us energy savings of 8 kW per plant. This will result in annual savings of almost EUR 50,000” reports Luc Ley as he looks his amortisation calculations. A period of five years had been forecast before a return on the investment would be achieved. Such a period will probably not apply in Luxembourg though, as the actual savings will not be 8 kW as expected, but are more likely to be 15 kW! And the corresponding longer maintenance intervals have not even been taken into account yet. Today, the wastewater treatment plant in Beggen is an example of highly available plant technology, jointly developed by users and manufacturers. Against the background of maximal operational reliability, the design was characterised by a healthy level of power reserves. According to Luc Ley, the confidence in AERZEN is “very high” as no other planning office had been involved in the retrofitting.